While you've reversed the order of the blocks specified, /fill is not capable of replacing specific blocks if the block you're placing is a tile entity (in this case, a chest). In those cases you can only specify NBT data for the tile entity:
/fill ~0 ~0 ~0 ~20 ~10 ~20 minecraft:chest 0 replace {LootTable:"minecraft:chests/HG_Loot_Table"}See this comment by Searge explaining that this isn't currently possible:
Replacing blocks with tile entities is not supported, this is due to a limitation in the parsing of commands. It may be changed in the future, but not until after 1.8.